1. Field of the Invention
The invention relates to a process and apparatus for stripping and regenerating fluidized catalytic cracking catalyst.
2. Description of Related Art
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree.-600.degree. C., usually 460.degree.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree.-900.degree. C., usually 600.degree.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking has undergone progressive development since the 40s. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and use of zeolite catalysts. A good overview of the importance of the FCC process, and its continuous advancement, is reported in Fluid Catalytic Cracking Report, Amos A. Avidan, Michael Edwards and Hartley Owen, as reported in the Jan. 8, 1990 edition of the Oil & Gas Journal.
Modern catalytic cracking units use active zeolite catalyst to crack the heavy hydrocarbon feed to lighter, more valuable products. Instead of dense bed cracking, with a hydrocarbon residence time of 20-60 seconds, less contact time is needed. The desired conversion of feed can now be achieved in much less time, and more selectively, in a dilute phase, riser reactor.
Although reactor residence time has continued to decrease, the height of the reactors has not. Although the overall size and height of much of the hardware associated with the FCC unit has decreased, the use of all riser reactors has resulted in catalyst and cracked product being discharged from the riser reactor at a fairly high elevation. This elevation makes it easy for a designer to transport spent catalyst from the riser outlet, to a catalyst stripper at a lower elevation, to a regenerator at a still lower elevation.
The need for a somewhat vertical design, to accommodate the great height of the riser reactor, and the need to have a unit which is compact, efficient, and has a small "footprint", has caused considerable evolution in the design of FCC units, which evolution is reported to a limited extent in the Jan. 8, 1990 Oil & Gas Journal article. One modern, compact FCC design is the Kellogg Ultra Orthoflow converter, Model F, which is shown in FIG. 1 of this patent application, and also shown as FIG. 17 of the Jan. 8, 1990 Oil & Gas Journal article discussed above. The compact nature of the design, and the use of a catalyst stripper which is contiguous with and supported by the catalyst regenerator, makes it difficult to expand or modify such units. The catalyst stripper design is basically a good one, which achieves some efficiencies because of its location directly over the bubbling bed regenerator. The stripper can be generously sized, does not have to fit around the riser reactor as in many other units, and the stripper is warmed some by proximity to the regenerator, which improves stripper efficiency slightly.
Although such a unit works well in practice, the regenerator operates with a relatively large catalyst inventory, a much larger catalyst inventory than would be required in a high efficiency regenerator. The long residence time, and relatively high steam partial pressure associated with single stage bubbling bed catalyst regeneration causes an undesirable amount of catalyst deactivation. We realized that it would be beneficial if the regenerator environment could be made drier, and/or if catalyst regeneration in such a regenerator could be conducted in stages, rather than in a single dense bed.
Some or our recent work has been directed to achieving multi-stage regeneration in such bubbling dense bed regenerators, such as our U.S. Pat. Nos. 5,032,251, 5,034,115 and 5,047,140 which are incorporated by reference.
Although all of the improvements listed above were in the right direction, they were not the complete solution. The approaches discussed above generally led to higher particulate loading than was desired in the dilute phase region above the bubbling dense bed. The approaches did not permit as much control as was desired in regard to the amount of catalyst recirculation to the coke combustor. Finally, we wanted to be able to achieve true multi-stage regeneration of catalyst, with complete CO combustion in the first stage, but only partial coke combustion. We had three goals:
1. Provide a simple and reliable way to control regenerated catalyst recycle to a fast fluidized bed coke combustor in an Orthoflow regenerator.
2. Have the benefits of high superficial vapor velocity first stage regeneration in a coke combustor immersed in a bubbling dense bed, without undue increase in dust loading in vapor space above the dense bed.
3. Incorporate a relatively fail safe method for achieving complete CO combustion, with only limited coke combustion, in the first stage of such a regenerator, without adding large amounts of Pt to the FCC catalyst inventory.
We developed several designs or modifications which reached the above goals.